Mehdi Mostefa's second-half strike gave Ajaccio all three points as they moved above Brest in the Ligue 1 table.
The Algerian's 55th-minute volley deflected off Brest defender Paul Baysse and into the top corner as Ajaccio extended their unbeaten run against today's opponents to 11 games.
Ajaccio are now on nine points after seven matches, the same as Brest, but are one place better off in eighth due to a superior goal difference in the early standings.
The hosts started well and could have been ahead as early as the ninth minute but Brest goalkeeper Alexis Thebaux was on hand to keep out Adrian Mutu's shot on the turn.
It was good work from Johan Cavalli to tee up the Romanian but Mutu was nearly made to pay for his profligacy when Bruno Grougi smashed a volley against a post from Ben Basat's left-wing cross.
That spurred the hosts into action and Mutu was put through on goal but was denied by a goal-line clearance with Thebaux beaten.
The former Chelsea and Fiorentina striker then narrowly missed the target from a free-kick just outside the penalty area, with Thebaux once again well beaten.
Brest started the second half in confident fashion, with Baysse heading inches over the bar from a Grougi corner.
The miss would prove costly as, two minutes later, the hosts broke the deadlock.
After Brest had only half cleared the ball, Mostefa pounced and thundered a volley into the top corner - via a deflection from the unfortunate Baysse.
The visitors did not have any meaningful opportunities to salvage a draw, with Jonathan Ayite's two blocked shots from long range the closest they came.
And they nearly fell further behind in the closing stages as a poor clearance from Baysse found Ajaccio midfielder Chakhir Belghazouani but the Moroccan's first-time strike produced a fine reaction save from Thebaux to keep the deficit to just one goal.